This invention concerns methods for increasing the temperature of heat to a higher temperature level. The transformation of heat is accomplished by a sorption system in which a working fluid is desorbed from a solid adsorbent at an intermediate temperature T.sub.1 and a relatively low working fluid pressure p.sub.0. The generated working fluid vapor is transformed to a liquid phse at a lower temperature T.sub.0 and the heat liberated in the phase change process is removed. The liquid phase of the working fluid is transformed to the gaseous phase at a higher working fluid pressure p.sub.12 by heating it to an intermediate temperature T.sub.1 '. The generated gaseous working fluid is adsorbed in a solid adsorbent and the heat of adsorption is liberated at a higher temperature T.sub.2. The process is repeated by switching the solid adsorbent from the adsorption to the desorption phase.
A heat transformer is a device which takes up heat at an intermediate temperature T.sub.1 (e.g. industrial waste heat), transforms a portion of the heat to a higher temperature T.sub.2 and release the remaining portion of the input heat at a lower temperature T.sub.0. The operation of the system is sustained by the heat input at T.sub.2 and electrical power is only required for auxilliary devices like liquid pumps or electrical controls.
The patent DE-PS No. 5796 57 describes a heat transformer which consists of an adsorption heat pump in which the directions of mass and heat flow are reversed. In this system an absorbent liquid (e.g. aqueous LiBr-solution) is used to absorb or generate working fluid vapor e.g. water vapor). The liquid absorbent with a high working fluid concentration is heated to an intermediate temperature T.sub.1 in a generator and working fluid vapor is generated at a low working fluid vapor pressure p.sub.0. The working fluid vapor is condensed in a condenser at a lower temperature T.sub.0 and the heat of condensation is rejected to the ambient.
The condensed working fluid is pumped into an evaporator at a higher working fluid pressure p.sub.1, where it is evaporated by the input of heat at an intermediate temperature T.sub.1 '. In a container, called absorbe, the working fluid vapor is adsorbed in said absorbent liquid, which is circulated from the generator to the absorber and back to the generator by mans of a second pump. The process is continued by supplying heat at T.sub.1, removing heat at T.sub.2 and T.sub.0 and by the operation of the two pumps.
Further heat transformers are described in the patent DE Nos. 10 20 997, 25 54 937, 26 49 441, 26 35 557 and 27 27 990 as well as from the proceedings of the International Congress of Refrigeration in Paris 1983 (page 55 ff.). The heat transformer described in the latter publication uses an aqueous LiBr-solution. LiBr-solution as absorbent liquid with water as working fluid is restricted to temperatures below 160.degree. C. since LiBr is very corrosive at higher temperatures and undergoes chemical reactions. Further the price of LiBr is rather high.
The same basic principle is used in an ammonia-water heat transformer, which is e.g. described in "Warmetransformator zur Prozesswarmeruckgewinnung aus Abwarme", Brauwelt (1983), p. 1508 ff. The use of ammonia-water is working fluid/absorbent combination limits the upper temperature of the process to 180.degree. C., since ammonia dissociates at high temperatures. The ammonia pressure in the evaporator reaches 109 bar for an output temperature of 130.degree. C. Further ammonia is poisonous, which limits the use of these systems (in conjunction with the high pressure) and increases the cost of such systems (due to safety equipment and very heavy tanks).
In a further system concentrated sulfuric acid is used as absorbent liquid. This system is described in the proceedings of the Int. Energy Conv Eng. Conference, Atlanta 1981, S. 926 ff. Despite the excellent results of such systems in the experimental stage the operation will be limited to special cases due to the dangers, handling problems and the corrosiveness of this combination.
A new development is the use of zeolite (a solid which is found as a mineral in volcanic rock or is produced synthetically) instead of a liquid absorbent in heat transformers.
A heat transformer using zeolite as solid adsorbent and water as working fluid is described in the German patent application DE-OS No. 30 22 284 (U.S. patent application No. 272,185). Since a solid adsorbent is used, the adsorbent cannot be pumped in a loop between the absorber and the generator as described above. Therefore a discontinuous operation is used: one or more containers filled with zeolite as solid adsorbent, these so-called adsorber-generators are alternately used as generator while supplying heat at an intermediate temperature T.sub.1 at a low pressure P.sub.0 and then used as adsorber to produce output heat at a higher temperature T.sub.2 at a higher pressure p.sub.1.
The solid adsorbent is cycled in temperature and pressure including the embedded heat exchangers and the container itself. This requires a certain minimum cycling time. The mass of adsorbent per adsorber/generator has to be large enough to provide output heat over one half cycle of the system. The cycling time has a lower bound due to the thermal masses involved and due to the high cost of additional heat exchanger area which is necessary for faster cycling. Therefore the mass of adsorbent required per adsorber/generator is quite large. An operation above 1 bar would require thickwalled pressure tanks. Below atmospheric pressure the adsorber/generators can be constructed as thin-walled containers with the solid adsorbent supporting. the sheet metal against th outside vapor pressure.
For economical reasons the pressure of solid adsorbent systems is limited to 1 bar. With water as working fluid this results in a maximum temperature of 100.degree. C. in the evaporator of the above-described heat transformer. Additionally, the use of zeolites is limited to a pressure of about 2 bar water vapor pressure due to the poor stability of most zeolites at higher pressures.
The use of a zeolite heat transformer according to the patent application DE-OS No. 30 22 284 results in comparatively low efficiency (c.o.p.) since the sensible heat of the solid adsorbent plus the heat exchangers and the container has to be generated by valuable heat of adsorption during each adsorption cycle. After the adsorbent is saturated at the temperature T.sub.2 and the working fluid pressure p.sub.1 the sensible heat of container, heat exchanger and solid adsorbent is (irreversibly) used in the desorption process for the desorption of working fluid which is condensed at a low temperature. The sensible heat between T.sub.2 and T.sub.1 uses up a significant portion of the total available heat of adsorption.